Interannual changes in radiocesium concentrations in annually laminated tufa following the Fukushima Daiichi Nuclear Power Plant accident

Abstract

Chemical and radiocesium concentrations in water and riverbed tufa of the Nigori River, flowing out of the southern foot of Mt. Asama volcano in central Japan, were investigated following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. This tufa is characterized by porous and dense laminae. Its annual lamination (varve) is formed by seasonal changes in dissolved CO2 concentration in water with temperature, and the tufa calcite is mostly precipitated during summer. We analyzed radiocesium concentrations in ∼4-mm-thick tufa samples that included four or five varves collected after the fall in 2011–2014. The activity ratio of 134Cs/137Cs in tufa collected in 2011 and 2014 was 0.96 ± 0.01 and 0.99 ± 0.01, respectively, which implies radiocesium generated as a result of the FDNPP. Sequential extractions indicate that 137Cs in tufa collected in 2011 is occupied in exchangeable and carbonate phases (55.2% and 44.8%, respectively). The 137Cs concentrations equivalent to tufa calcite that were precipitated after 2011 showed variations similar to the amount of summer rainfall during 2011–2014, which may be attributed to increases in dissolved 137Cs concentrations in river water associated with intensified rainfall and microbial action over the summer. In contrast, the 137Cs sorbed to tufa calcite was not retained in the riverbed, and its migration may be attributed to the porous texture of tufa and the high flow rate of the river. Therefore, tufa calcite is easier to dissolve even though it adsorbs compared with other minerals (mica, andosol etc.).